Rack Mount PC

i want to rack mount my PC. i'm a musician and i use my PC for audio applications only. to fit it in my rack with my samplers and synths would be very handy and keep things more organized in my studio.

shopping around i see stores offering 19" rackmount cases. The rack i use (again, for music modules) has a width from mounting strip-to-mounting strip of 19", but the actual width of gear in the rack is about 17 1/2". are these 19" cases going to fit in my rack?

also, what in the world am i shopping for? any suggestions on where to buy it? i'm finding lots of stores that offer pre-built rack PCs, but i just want the case. some stores offer custom-made cases, but that shouldn't be necessary, should it? i also find stores selling Server Blades and Raid Array cases, but i just want a simple PC case to fit:

ATX motherboard
7 PCI/AGP slots
3 HDD
1 FDD
1 CDROM
a couple fans

it looks like 4U is the smallest i could get, yes? how much interior room will that 4U leave me? i may want to add some LEDs or other mods later.